Fundamental processes of dye chemistry. 1949

192 INTERMEDIATES

polysulfonie acids. Only a few of the typical reactions will be described in detail here in order to give the beginner a picture of this field of intermediates.

The first product formed by the actiori of concentrated sulfuric acid on^-naphthol is 2-naphthol-l-sulfonic acid (page 199). This compound, however, is very unstable and rearranges, even in the cold, in the presence of excess sulfuric acid, to form 2-naphthol-8-sulfonic acid (Bayer acid, croceine acid), and this in turn rearranges, only partially in the cold but completely at higher temperatures, into 2-naphthol-6- sulfonic acid (Schaeffer acid).

When an excess of acid is used, there are always formed also some 2-naphthol-3,6-disulfonic acid and 2-naphthol-6,8-disulfonicacid. These disulfonic acids usually react with diazonium salts to yield, respectively, red and yellow dyes, and are therefore known commonly as R acid (E.=rot) and G acid (G=gelb). R and G acids are the main sulfonation products if a large enough excess of sulfuric acid is used, relatively more of the R acid being formed at low temperatures and more G acid at higher temperatures. Under very vigorous conditions (oleum at elevated temperatures), both disulfonic acids are converted to 2-naphthol- 3,6,8-trisulfonic acid. All of these acids are important starting materials in the azo dye industry. In most cases, it is practically impossible to prepare any one of them separately; it is necessary, as a rule, to separate them from mixtures.

S08H 1	S03H
/\ -OH	OH
	Bayer acid
	Croceine acid

HCLS-I

HO,

SULFONIC ACIDS OF	NAPHTHALENE	193
Sulfonation reactions, especially those carried out at low tempera-
tures, often form, in addition to the	above acids, appreciable quan-
tities of 2-naphthol-l,6-disulfonic acid	and 2-naphthol-l,3,6-trisulfonic

acid. Since these products have no technical value, the sulfo group in the 1 position is split off by diluting the sulfonation mixtures with water and heating for 0.5 to 1 hour at 90-100°.

Separation of the individual sulfonic acids from a reaction mixture is based on the different solubilities of their alkali salts. The three acids which are technically the most important and also which are most frequently formed simultaneously are the 2,6-monosulfonic acid (Schaeffer acid), and the 2,3,6-, and 2,6,8-disulfonic acids (R and G acids). Of these, the first gives a sodium salt having the lowest solubility in cold water, and the presence of salt has little effect on its relatively low solubility. To the contrary, the sodium salt of R acid is much more soluble in pure water, but is largely precipitated by a relatively small amount of salt. The sodium salt of G acid is easily soluble in highly concentrated salt solutions; on the other hand, its potassium salt is only moderately soluble in cold water and still less soluble in potassium chloride solutions. The foregoing statements apply to the neutral salts in which all of the sulfo groups are neutralized but the hydroxyl group is free. Notwithstanding statements in the literature to the contrary, the salts which separate from neutral or acid solution are always these neutral salts and never the acid salts in which free sulfo groups are still present. In alkaline solution, however, basic salts are formed, in which the hydroxyl group is also neutralized. These basic salts are more easily soluble than the neutral salts and are not suitable for isolation. Separation of mixtures, therefore, is always carried out in neutral or acidic solution.

In practice, the neutral or slightly acid solution of the sodium salts, containing little or no sodium chloride, is evaporated to a definite volume which depends on the composition of the reaction mixture and which is selected so that on cooling only the sodium 2-naphthol-6-sulfonate crystallizes out, but this as completely as possible. The precipitate is removed, and, to the filtrate, usually after further evaporation, is added enough salt to make it an 18 to 20 per cent salt solution. R salt then precipitates when the solution is cooled. Potassium chloride is then added to the mother liquor to precipitate the potassium salt of G acid.

In order to obtain really pure products from a given reaction mixture, not only is it necessary to have exactly the correct concentration and quantity of salt, but the filtration and washing of the separate precipitates must be done with exacting care in order to remove the

194	INTERMEDIATES

mother liquor as completely as possible with a minimum of wash liquid. It should be emphasized that /?-naphthol should always be finely pulverized before use.If this precaution is not taken, part of the naphthol is sulfonated but the large lumps floating around in the mixture are not attacked and the results are completely unsatisfactory. This is also the case if the reaction mixture is merely allowed to stand and is not stirred continuously. The apparatus used is the same as that used

for the preparation of naphthalene-/?-sulfonic acid (Fig. 19).

2-Naphthol-6-8ulfonic Acid (Schaeffer Acid)

(R Acid and G Acid)

To 250 grams of concentrated sulfuric acid (66° Be) is added, with stirring, 144 grams (1.0 mole) of finely powdered, pure /3-naphthol. The temperature rises to about 30°C. The mixture is heated for 8 hours in a boiling water bath, and then poured while still warm into 1 liter water. The diluted solution is held at 95-100° with stirring for about 1 hour. If necessary, water is added so that the total volume does not become less than 1 liter. An additional 500 cc. water and 100 grams of anhydrous sodium sulfate (or the corresponding amount of the hydrated salt) are then added, and the solution is neutralized at about 90° by the addition of chalk (about 190 grams is required). The chalk must be added very carefully in small portions, particularly toward the end of the neutralization, in order to prevent foaming over. The mixture is finally boiled and filtered hot to remove the calcium sulfate which is washed carefully with hot water until a portion of the washings, treated with soda, gives only a weak coloration with diazotized aniline. The combined filtrate and washings are heated to boiling and freed from residual calcium compounds by the addition of enough anhydrous soda (about 30 grams) to make the solution barely alkaline to phenolphthalein. (Complete removal of the calcium is necessary only if it is important that the product dissolve to form a clear solution even in the presence of soda.) At this point, a filtered test portion of the filtrate should give no further precipitation when more soda is added. The precipitated calcium carbonate is filtered off and washed with hot water, and the filtrate is made just acid with concentrated hydrochloric acid (about 40 cc.) and evaporated to 1 liter. The concentrated filtrate is cooled with stirring, during which the volume decreases still more by evaporation; after cooling, the volume should be 800 cc.

The mixture is allowed to stand overnight, and the resulting thick

SULFONIC ACIDS OF NAPHTHALENE

195

paste is filtered by suction. In this filtration, the solid must be pressed out thoroughly, using a pestle or the flat surface of a glass stopper, to prevent the formation of cracks or perforations in the filter cake and to ensure as complete removal of the mother liquor as possible. When no more drops of filtrate are formed, the vacuum is broken and 50 cc. cold water is poured into the funnel. Again, care is taken that no canals are formed through which water can pass without effective washing, and that the wash water works its way uniformly through the precipitate. Vacuum is reapplied, slowly at first and then as fully as possible, and the solid is pressed out thoroughly again. Washing with 50-cc. portions of cold water is repeated twice more. If the operations are carried out properly, the final precipitate contains such a small quantity of liquid that pressing in a screw press can be omitted unless large quantites are being worked with. The product, after drying in a steam heated oven, consists of 150-155 grams of technically pure sodium 2-naphthol- 6-sulfonate, 1 gram of which uses 36.5to 37 cc. 0.1 N diazo solution.

The yield is about 56 per cent of the theoretical amount, and the product is usable for most purposes without further treatment. The last traces of disulfonic acids can be removed, if desired, by recrystallization from 3 to 4 parts of hot water.

The combined filtrate and wash water from the monosulfonate are evaporated to a volume of 300 cc., and 40 grams of salt is added to the hot concentrated solution. The mixture is allowed to stand overnight, and the resulting precipitate is filtered off and washed three times with 20-cc. portions of a half-saturated salt solution (mixture of equal parts of saturated salt solution and water), following the same filtering procedure used with the first precipitate. The product, after being dried in a steam heated oven, consists of 50 to 55 grams of crude R salt, of which 1 gram corresponds to 23 to 26 cc. Q.I N diazonium solution. Thus, the yield is 12.5 to 13per cent of the theoretical amount. The purification of this product is discussed later.

The filtrate from the R salt is heated to boiling and 60 grams of potassium chloride is added. The solution is cooled and allowed to stand overnight. The copious precipitate is filtered off and washed, as described above, with 10 per cent potassium chloride solution. The product, dried in a steam heated oven, weighs 40 to 45 grams, and consists chiefly of G salt. One gram uses 23 to 25 cc. 0.1 N p-nitrodiazo- benzene solution. The yield is about 10 per cent of the theoretical amount.

196 INTERMEDIATES

The last mother liquor still contains about 12 per cent of the starting material in the form of sulfonic acids which can be titrated with p-nitrodiazobenzene solution. Besides residual Schaeffer salt, R salt, and G salt, other isomers are present which have not been investigated.

The crude R salt obtained in the above procedure always contains some Schaeffer salt. The amount of the impurity should not be more than 5 per cent if the work has been done carefully. The crude R salt is usable as such for many purposes, but, if desired, a purified material can be obtained in the following way.

50 grams of the crude product is dissolved in 250 cc. boiling water and 25 grams of salt is added. The solution is then filtered to remove a slight turbidity and placed in the ice chest overnight. The resulting precipitate is filtered off with suction and washed with three portions, totaling 50 cc., of a mixture of 1 part of saturated salt solution and 3 parts of water. In this way, 65 to 70 per cent of the crude product is obtained as pure R salt if the crude product did not contain more

than 5 per cent of the other sulfonic acids. In other cases,	the	purification must
be repeated.
The filtrate is evaporated to half its original volume	(i.e.,	to 125 cc.). On

cooling, about 25 per cent of the original crude material crystallizes out. This second fraction is mainly Schaeffer salt which can be obtained in a pure state by recrystallizing from 4 parts of water. The mother liquor from the second fraction gives a precipitated mixture of Schaeffer and R salts on addition of salt. This material can be separated into its constituents by repeating the operations described.

The fraction obtained by precipitation with potassium chloride is G salt which contains 5 per cent, at the most, of R salt as an impurity. In the laboratory, this crude G salt can be purified by recrystallization from water. The method used in the plant is based on the fact that G salt couples with diazonium compounds much less readily that either R salt or Schaeffer salt. A test is made to determine the content of easily coupling sulfonic acids by titrating a sample with diazobenzene or diazoxylene in dilute solution. The crude G salt is then treated with the correct amount, as indicated by the titration, of a slowly reacting diazo compound, usually diazoxylene. Under these conditions, only the impurities couple, leaving a solution of pure R salt after the dye which is formed is removed. The G salt solution can be used directly in the preparation of azo dyes. The dye from the impurities is sold under the name Ponceau.

The products can be tested for purity by determining their fluorescence and their behavior with diazotized p-aminoacetanilide (acet-p-phenylenediamine). Pure Schaeffer salt in a neutral solution exhibits no preceptible fluorescence; its alkaline solutions have a violet-blue fluorescence of low intensity. R salt and G salt both show greenish blue fluorescence, weak in neutral solution, but very intense in alkaline solution. A slight contamination of Schaeffer salt by either R or G salt is sufficient to mask the fluorescence of the Schaeffer salt. When the products are colorless, the presence of small amounts of impurities in Schaeffer salt can be detected in this way, and by comparison with known mixtures the amount of contamination can be estimated. When the crude products are strongly colored, however, the sensitivity and accuracy of this test are greatly reduced.

Schaeffer salt reacts with diazotized p-aminoacetanilide to give an easily soluble orange red dye which is precipitated only by the addition of a large amount of salt. The dye from the same diazo compound and R salt, on the other hand, is almost completely precipitated, by very small amounts of salt, in the form of lustrous bronzy crystals which appear bluish red by transmitted light; the filtrate has only a light bluish red color. The presence of Schaeffer salt is shown by a

SULFONIC ACIDS OF NAPHTHALENE

197

stronger, more yellowish color in the filtrate. In dilute solution, slightly alkaline with carbonate, G salt does not couple, or couples very slowly, with diazotized p-amino- acetanilide. Hence, in testing a sample of G salt, an immediate formation of a deep red coloration shows the presence of an impurity, usually Schaeffer or R salt. The presence of G salt in products consisting mainly of the other sulfonic acids is shown by the intense G salt fluorescence in the filtrate after the dye, formed from the product and diazotized p-aminoacetanilide, has been salted out and filtered off. This filtrate shows no further coupling reaction with diazotized p-aminoacetanilide, but does react to form dye with more energetic diazo compounds, e.g., that from p-nitroaniline.

The titration of Schaeffer salt and R salt is done most satisfactorily with diazotized p-aminoacetanilide in slightly alkaline solution (carbonate), that of G salt with diazotized p-nitroaniline in the presence of bicarbonate. In the case of

mixtures, the difference between the two titrations gives a measure of the		amount
of G salt	present.
The	analysis of mixtures of Schaeffer salt and R salt is described	in the
Analytical	Section.
2-Naphthol-6,8-disulfonic Acid (G Acid) and 2-Naphthol-
	3,6-disulfonic Acid (R Acid)
If G acid is desired as the chief product, the reaction is effected

with a larger quantity of sulfuric acid and higher acid strength, at a moderate temperature, in no case over 60°C.

To 200 grams of 100 per cent sulfuric acid, well stirred and cooled in running water, is added 72 grams (0.5 mole) of pure, finely pow- dered /?-naphthol at such a rate that the temperature does not rise above 20°. The mixture is stirred at room temperature for 1 hour, after which a test sample should form a clear solution with water and the diluted solution should show no turbidity on heating. Again with cooling, 100 grams of 20 per cent oleum is added slowly, while the temperature is not allowed to rise appreciably above 20°. The reaction mixture is now heated in a water bath to 55-60° and stirred at this temperature for about 40 hours. At this point a test sample, diluted with water and made slightly alkaline with soda, is treated with diazotized p-aminoace- tanilide. Only the easily salted out, blue red, R salt dye should be formed, and none of the more soluble orange red dye from Schaeffer salt (see preceding section). Toward the end of the sulfonation, a crystalline precipitate usually separates, and the reaction mixture becomes a thick paste which is, however, easily stirred. The mixture is poured into 1 liter water, 75 grams of anhydrous sodium sulfate is added, and the solution is stirred for 1 hour at 95°, replacing the water lost by evaporation. By this treatment, any 2-naphthol-l,6-disulfonic acid and 2-naphthol-l,3,6-trisulfonic acid are converted to Schaeffer

198 INTERMEDIATES

acid and R acid, respectively. The hot solution is then neutralized by adding about 300 grams of chalk with stirring. This addition can be made rather rapidly without danger of foaming over. Filtering and washing of the calcium sulfate precipitate, removing the remaining calcium compounds by precipitation wtih soda ash (about 25 grams), and reacidifying the filtrate with concentrated hydrochloric acid (about 30 cc.) are carried out exactly as in the preparation of Schaeffer acid. Finally, the solution is evaporated to 400 cc., treated hot with 60 grams of salt, and allowed to cool with stirring. Most of the R salt, and some Schaeffer salt which is present, crystallize out on standing overnight. The precipitate is filtered off and washed with half-saturated salt solution, proceeding exactly as described in detail for the isolation of Schaeffer salt (page 194). The dried product weighs about 65 to 70 grams. Titration with diazotized p-aminoacetanilide (120 cc. 1 N solution) and with diazotized p-nitroaniline (150 cc. 1 N solution) shows that the product corresponds to about 30 per cent of the ^-naphthol used and contains about 80 per cent R salt with a very small amount of Schaeffer salt, and 20 per cent G salt. Pure R salt can be obtained by the method described on page 196.

The filtrate is reheated to boiling, treated with 100 grams of potassium chloride, allowed to cool with stirring, and then left overnight. The precipitate is filtered off,washed thoroughly with three 50-cc. portions of 10 per cent potassium chloride solution, and dried in a steam heated oven. The product weighs 140 to 150 grams and corresponds to about 300 cc. 1 N solution of diazotized p-nitroaniline. Thus, the yield is about 60 per cent of the theoretical amount. In carefully conducted preparations, the colorless product is pure G salt which gives no immediate dye formation with diazobenzene in weakly alkaline, dilute solution.

The mother liquor still contains about 10 per cent of the /3-naphthol in a complex mixture with various sulfonic acids which cannot be separated in any way which is feasible technically.

2-Naphthylamine-l-8ulfonic Acid (Tobias Acid)

from /?-Naphthol

	S03H	SO3H
OH	OH	NH,
		NH,

+ (NH4)8S08

SULFONIC ACIDS OF NAPHTHALENE

199

(a) 2-Naphthol-l-sulfonic Acid from p-Naphthol

In a 1-liter, five-necked flask, fitted with a stirrer, preferably of the paddle type, operating through a mercury seal in the center neck, a dropping funnel, a thermometer, a gas inlet tube, and a gas outlet tube, 72 grams (0.5 mole) of dry /3-naphthol is treated with 280 grams of hot (100°C.) completely dry nitrobenzene. (The simplest method for drying nitrobenzene is to distill and discard the first fraction which contains all of the water.) The resulting completely clear solution is allowed to cool with stirring, part of the naphthol crystallizing out. The mixture is cooled in ice, and 35 cc. (62 grams) sulfuric acid-free chlorosulfonic acid is dropped in slowly, while the temperature is kept below 5°. The hydrogen chloride which is formed is passed through a calcium chloride tube into a suitable absorption apparatus. The /2-naphthol goes into solution rapidly, and toward the end of the reaction part of the naphtholsulfonic acid precipitates. When all of the chlorosulfonic acid has been added, the remaining dissolved HC1 is swept out by means of a stream of dry air.

Chlorosulfonic acid takes up water readily, decomposing into sulfuric and hydrochloric acids. Therefore, it must be kept in containers having absolutely tight ground glass stoppers. Ordinary glass stoppers are not usually sufficiently tight for this purpose. If the chlorosulfonic acid is not known to be of the highest quality, it should be distilled before use, carrying out the distillation in the complete absence of moisture.

The nitrobenzene solution, together with the suspended solid material, is shaken with a mixtureof 300 cc. water and 100grams of ice, then twice with 100-cc. portions of cold water. The combined aqueous extracts are filtered through a moistened paper and then saturated with 200 grams of salt, whereupon sodium 2-naphthol-l-sulfonate is precipitated in colorless plates. Stirring is continued until all of the salt is dissolved, and the mixture is allowed to stand overnight, then filtered. The product is pressed out and washed twice with saturated salt solution. The filter cake is used directly for the preparation of 2-naphthyl- amine-1-sulfonic acid after tests 'have shown that it does not fluoresce in soda solution and does not yield a water soluble, orange red dye with diazotized aniline and thus does not contain isomeric naphtholsulfonic acids.

If it is desired to isolate the sodium sulfonate in a pure state, the filter cake is dissolved in a small amount of cold water, the solution is extracted with several portions of benzene to remove nitrobenzene, then filtered and the product again

200	INTERMEDIATES

precipitated with salt, filtered off, washed with saturated salt solution, pressed out, and aried. This yields a material which is pure except for the presence of salt, which can be removed if desired by crystallization from alcohol.

The product is analyzed by boiling a weighed sample under reflux for 2 hours with 10 parts of 20 per cent sulfuric acid. The sulfo group is split off by this treatment. The resulting /3-naphthol is dissolved by adding dilute sodium hydroxide and titrated with 0.1 N diazobenzene solution.

(b) 2-Naphthylamine-l-sulfonic Acid from 2-Naphthol- 1-siilf onic Acid

The filter cake of sodium 2-naphthol-l-sulfonate, obtained under (a), is placed in an enameled autoclave of about 1-liter capacity with 250 cc. 20 per cent ammonia and an ammonium bisulfite solution prepared by saturating 60 cc. 20 per cent ammonia with SO2 with cooling. The mixture is heated in the autoclave for 8 hours at 145-150°C. (internal temperature). In large scale preparations, the ammonia is then distilled off and reused in the next run. It is more convenient in laboratory preparations to saturate the solution with salt, whereupon the sodium salt of 2-naphthylamine-l-sulfonic acid is precipitated almost completely. The mixture is allowed to stand overnight, and then the sulfonate is filtered off and washed well with saturated salt solution to remove ammonia and ammonium salts. The resulting filter cake is dissolved in hot water and the solution is filtered and treated with sufficient concentrated hydrochloric acid to make it distinctly acid to Congo red. After cooling the solution thoroughly, the precipitate, consisting of the free 2-naphthylamine-l-sulfonic acid in the form of needles, is filtered off, washed with cold water, and dried. The yield is 98 grams, or 83 per cent of the theoretical amount based on /2-naphthol.

As mentioned earlier (page 192), 2-naphthol-l-sulfonic is the first product formed by the reaction of concentrated sulfuric acid on /3-naphthol. It isrearranged so rapidly by the unavoidable excess of sulfuric acid, however, that it cannot be prepared by direct sulfonation with sulfuric acid.

The procedure described above involves sulfonation in the absence of free sulfuric acid at low temperatures, 'and avoids the use of an appreciable excess of the sulfonating agent. Similar procedures are generally applicable to the preparation of sulfonic acids which cannot be prepared satisfactorily by the ordinary sulfonation methods, because they are easily isomerized or sulronated further by concentrated sulfuric acid. Even compounds which are ordinarily converteddirectly into disulfonic acids, such as carbazole, 4,4'-dihydroxydiphenylmethane, etc., can be* monosulfonated in this way.

2-Naphthol-1-sulfonic acid does not couple with diazo compounds under ordinary conditions,because the single reactive position in the 0-naphthol nucleus is occupied. However, at higher temperatures and in acid media, the sulfo group is replaced by the azo radical and the dye that is formed is the same as that from

SULFONIC ACIDS OF NAPHTHALENE

201

/3-naphthol. This property is made use of in developing paranitraniline red on the

fiber.ei

The chief use of 2-naphthol-l-sulfonic acid is as an intermediatein the preparation of 2-naphthylamirie-l-sulfonic acid. The latter, when diazotized and coupled with /3-naphthol, yields the valuable lake dye, lithol red R, which is used in large

quantities.

The conversion of 2-naphthol-l-sulfonic acid'into the corresponding naphthylaminesulfonic acid is another example of the Bucherer reaction which was discussed in connection with the preparation of l-naphthol-4-sulfonic acid (page 182).

l-Amino-2-naphthol-4-sulfonic Acid from /2-Naphthol

Preparation of an Aminonaphthohulfonic Acid from the Hydroxynitroso Compound (Quinonemonoxime)

(a) NitrosO'p-naphthol. In a 3-liter beaker, 72 grams (0.5 mole) (the preparation starting with 1mole would require too large a volume) of ^-naphthol is dissolved at 50°C. in a mixture of 65 grams of 35 per cent sodium hydroxide solution and 750 cc. water. To this solution, which should have a distinct, but weak, reaction to thiazole paper, are added 36 grams of 100 per cent sodium nitrite and enough water and ice to bring the volume to 1.5 liters and to lower the temperature to 0°. About 160grams of 40 per cent sulfuric acid is then added over a period of three hours with good stirring. The solution should become distinctly acid to Congo red and show a positive reaction with starchiodide paper. After at least 1 hour, the nitrosonaphthol is filtered off on a large suction funnel and washed thoroughly. It is chemically pure provided that pure /3-naphthol was used.

(6) Reduction and rearrangement to aminonaphtholsulfonic acid.

The moist nitrosonaphthol is stirred with a small amount of water to form a uniform paste which is cooled in ice to 5°C., and 320 grams of sodium bisulfite solution (about 25 per cent SO2 ) is added in one portion. (It is essential that the bisulfite solution be titrated to determine the SO2 content. For each mole of /}-naphthol, 2.5 moles SO2 are used.) The nitrosonaphthol goes into solution in a short time, but if necessary, a small amount of dilute sodium hydroxide is added carefully.

The solution is filtered to remove any tarry material which it contains. (The hydroxylaminesulfonic acid may be salted out to yield the Alsace green J or Dioxine N of the trade, a dye which is used to a certain extent in calico printing. Its iron, lake is very fast to light.)

01 The Calico Printers Assoc. and E. Fourneaux, Ger. Pat. 204,702 (1909) [FrdL, 9, 408 (1908-1910); C.A., 3, 958 (1909)].

202	INTERMEDIATES
	NO
,/X/VoH
\/\/	SO3Na	Quinoneoxime
	I
	NOH	NH2
NaHSO?	rOH 4"NaHS03(SOa) ( Y'V"011
	"Dioxine"	S03H
		l-Amino-2-naphthol-
		4-sulfonic acid
The filtered solution, about 1 liter		in volume, is transferred to a

beaker, and a mixture of 100 grams of 66° Be sulfurie acid and 200 grams of water is added at 25°C. The solution should now have a strong mineral acid reaction. After 1 hour, the solution is heated to 50°, then allowed to stand overnight. The reaction mixture solidifies to a cake of the free aminonaphtholsulfonic acid, which is filtered off and washed thoroughly with water. The yield is about 90 per cent based on the /2-naphthol used.

The diazotization of l-amino-2-naphthol-4-sulfonie acid is described on pages 242 and 248; an alternate method for preparing the same diazo compound js given on page 178. Coupling with 0-naphthol yields palatine chrome black 6B62 (eriochrome blue-black R, salicine black U), first discovered by Badische A.S.F., and independently and almost simultaneously by Geigy (Sandmeyer) and by Kalle (Elbel). Coupling with a-naphthol gives eriochrome blue-black B63, discovered by Geigy. Both of these dyes are very fast blue-black chrome dyes. It is interesting that the coupling reaction with a-naphthol in strongly alkaline solution involves only the position ortho to the hydroxyl group.

The diazo compound of l-amino-2-naphthol-4-sulfonic acid is so stable that it can be dried without danger, and can be nitrated in concentrated sulfurie acid solution with mixed acid.64 The nitrated diazo compound reacts with the two naphthols to give the commercial chrome black wool dyes, eriochrome black T and A,65 which are cheap and almost unsurpassed for fastness.

The above method of sulfonation with sulfurous acid finds further application in the preparation of p-aminophenoldisulfonic acid from nitrosodimethylaniline and sodium bisulfite. In the rearrangement to the disulfonic acid, the dimethylamino group is split off with the formation of the p-aminophenol derivative. Pure dimethylamine is formed in the reaction.

62 Badische A. und S. F., Ger. Pat. 156,440 (1904) [FrdL, 8, 656 (1905-1907)].

63Geigy, Ger. Pat. 181,326 (1904) [Frdl, 8, 668 (1905-1907); C.A., I, 2329 (1907)].

64Sandmeyer and Hagenbach (Geigy), Ger. Pat. 164,655 (1905) [FrdL, 8, 647 (1905-1907)].

«« Geigy, Ger. Pat. 169,683 (1906) [FrdL, 8, 673 (1905-1907)].

	SULFONIC ACIDS OF NAPHTHALENE			203
(CH3)2	(CH3)a	OH
N	N
		-f- NaHSO3	+	NH(CHS),
	NO	NHa

/MVaphthylamine from ^S-Naphthol

2 NH2

\

A mixture of 144 grams (1.0 mole) of pure /?-naphthol and 600 grams of ammonium sulfite solution is heated in an autoclave equipped with a stirrer and heated in an oil bath. (The ammoniumsulfite solution can be prepared by saturating 250 grams of 20 per cent ammonia with SO2, then mixing the resulting solution with an additional 250 grams of ammonia.) To this mixtureis added 125 grams of 20 per centammonia, and the charge is heated 8 hours at a temperature (internal) of 150°C. and a pressure of about 6 atmospheres (steel tube manometer!). The autoclave is allowed to cool and the resulting cake of /?-naphthylamine is ground up in a mortar and washed thoroughly with water in a suction funnel. The ammonium sulfite solution can be used over again. The washed baseis dissolved in a warm solution of 110 grams of hydrochloric

acid	(containing no sulfurie acid) and 1.5 liters water, and	the solu-
tion	is filtered to remove some unreacted ^-naphthol. The	filtrate is

mixed with a solution of 100 grams of anhydroussodium sulfate in 250 cc. water to precipitate the /?-naphthylamine as the sulfate. The mixture is allowed to stand overnight,then the precipitate is filtered off and washed well with cold water. This product, after being dried, can be used directly for many purposes (see page 205).

To prepare the free base, the sulfate is mixed with 1 liter water and treated with 60 grams of soda ash dissolved in a small amount of water. The reaction requires several hours because of the low solubility of the sulfate, but it can be accelerated by continuousstirring and heating to 80°. The base is filtered off, washed, and dried at 80°. The yield is about 130 grams, or 85 to 95 per cent of the theoretical amount.

Technical Observations. It is absolutely necessary to use an autoclave heated' by an oil bath or steam jacket for such reactions. The naphthylamine separates out at the bottom of the container as an oily layer, and unless an oil bath is used, overheating occurs to such an extent that, in spite of the stirring, a large part of the product is converted to dmaphthylamine and decomposition products. The same conditions prevail in the preparation of a-naphthol (page 180).

204	INTERMEDIATES
/3-Naphthylamine is usually vacuum distilled in the plant. The distillation must
be done very carefully, since the compound decomposes easily. For		sulfonation
reactions, if the free	base is not isolated, the thoroughly dried sulfate,	mixed with

1 per cent soda (see also primuline), is added to sulfuric acid or oleum.

The Bucherer method has completely replaced the older procedure of heating the naphthol with ammonia. This process gave only about a 70 per cent yield at pressures of 50 to 60 atmospheres. The Bucherer reaction was discussed in more detail on page 182.

2,8,6- and 2,5,7-Aminonaphtholsulfonic Acids (Gamma and

J Acids) from /3-Naphthylamine

These two important sulfonic acids can be prepared in various ways. Three methods are used for preparing y acid, while J acid, so far as is known, is prepared only from naphthylamine, although by varying procedures. The starting material in all cases is ^-naphthol; the reactions involved are shown in the following scheme. We shall describe here only the preparations starting from ^-naphthylamine. The processes starting from /?-naphthol and its sulfonic acid derivatives are described briefly in the technical observations following the preparation

of J acid.

OH

OH

H03S

2,8-Dihydroxynaph- thalene-6-sulfonic acid

NH2

2-Amino-8-naphthol-6- sulfonic acid (Gamma acid)

OH

2-Amino-5-naphthol-7- sulfonic acid (iso-Gamma acid, J acid)

CO""\y\/1

SULFONIC ACIDS OF NAPHTHALENE

205

(a) 29698- and 2,5,7-l\aphthylaminedisulfonic Acids from {3-Naphthylamine

To 800 grams of 15 per cent oleum is added, over a period of 10 minutes, 192 grams (1.0 mole) of finely powdered ^-naphthylamine sulfate which has been intimately mixed with 1 gram of anhydrous carbonate. The temperature should not be allowed to rise above 50°C. The reaction mixture is now tested for complete solubility in water and soda; monosulfonation is usually complete in 15 minutes. The mixture is cooled to 40°, and 350 grams of 66 per cent oleum is added over a period of 15 minutes with continuous stirring. The sulfonation is continued for 1 day at 55°, then for an additional day at 85°. Under these conditions, the 2,5,7- and 2,1,5-naphthylaminedisulfonic acids, which are formed first, are converted to 2-naphthylamine-l,5,7-trisulfonic acid:

HO,S'7 Y \NH2 H03S/

	^S03H
	NH2
HOSS f^	S08H
	NH2
	HOaS

On completion of the sulfonation (which can be carried out more or less rapidly according to various procedures), the reaction mixture is cooled to 50°C. and poured into a stirred mixture of 800 grams of water and 1100 grams of ice. The final temperature should be about 60°. Pure 2-naphthylamine-6,8-disulfonic acid precipitates and is filtered off after the mixture has been held for 6 hours at 20°. The filter cake weighs about 210 grams (corresponding to 30 grams of NaNO2).

- SOoH

SOgH

206	INTERMEDIATES

In the dye industry, it is customary to express quantities in terms of nitrite equivalents. Since one gram molecule of any given amine requires 69 grams of NaNC>2 ( equivalent to 1 mole of nitrous acid ) in diazotization, yields of amines are commonly stated as corresponding to x per cent or x grams of nitrite. Thus, if a given quantity of naphthylaminesulfonic acid reacts with 17 grams of nitrite, the quantity is often described as that which is equivalent to 17 grams of nitrite.

Most technical recipes for azo dye preparation call for 1 gram mole or 1 kilogram mole of an amine, since then the quantities of nitrite, acid, and alkali to be used are always the same. Hence, to the technical chemist, the expression: "the yield corresponds to 35 grams of nitrite," means that the actual yield is about 50 per cent.

The filtrate, having a volume of about 2300 cc., is transferred to a dish, heated to boiling (125°), and held at this temperature for 4

hours*	(replacing the evaporated water), whereby the 1 sulfo group
is split	out;
	SO3H
		125°
	S03H	S03H

The solution is held for 2 days at 0° during which the 2,5,7 acid separates. The product, after filtering and pressing, weighs about 170grams. The mother liquor, which is about 40 per cent sulfuric acid, contains about 22 grams of sulfonic acids per liter, which is not recovered. The 2-naphthylamine-5,7-disulfonic acid corresponds to about 26 grams of

NaNO2.

The filter cake of the 2,6,8 acid is dissolved in 700 cc. boiling water and treated with 70 grams of salt. The voluminous precipitate of the monosodium salt of the pure acid makes the mixture almost solid. After 12 hours, the mass is broken up and filtered. The crude 2,5,7 acid is dissolved in 850 cc. hot water and reprecipitated in the same way with 85 grams of salt.

The two pure sulfonic acids are distinguishable by their very characteristic fluorescence, which is blue for the 2,6,8 acid and green for the 2,5,7 acid. The latter is clearly observable only with a very pure product and is masked by a small amount of the 2,6,8 acid. Furthermore, the two acids behave differently with an acetic acid solution of diazotized p-nitroaniline. The 2,6,8 acid gives, in dilute solution, only a light yellow coloration due to a diazoamino compound. The 2,5,7 acid, on the other hand, yields a red azo dye immediately. In either case, it is possible to estimate the purity of the product from the depth of color formed. The diazotized 2,6,8 acid couples with R salt to give a difficultly soluble, red azo dye which precipitates even in highly dilute solution and which dissolves on boiling to

ive a red color. The 2,5,7 acid, under the same conditions, yields an orange red fye which is very easily soluble.

* The solution can also be heated under reflux.

SULFONIC ACIDS OF NAPHTHALENE	207
(b) 2>Amino-8-naphthol-6-sulfonic Acid (Gamma Acid)
OH
HO.S
Alkali fusion of pure naphthylaminedisulfonic acids offers no	diffi-

culty, although it is desirable that the material be as free from salts as possible. Pure, dry 2-naphthylamine-6,8-disulfonic acid in an amount equivalent to 35 grams of nitrite (or the corresponding amount of moist material), 220 grams of chlorate-free sodium hydroxide, and 120 grams of water are heated in an autoclave (with stirrer) for 7 hours at 205210°C. The pressure increases to 14 atmospheres. After cooling, the pressure is released and the contents of the autoclave are diluted to 1 liter with water (the solution should not smell strongly of ammonia), and made strongly acid with concentrated sulfuric acid. About 250 grams of acid is required. The mixture is filtered after several hours, and the precipitate washed thoroughly with cold water. The y acid is quite insoluble in water. It is pressed out and dried at 100°. The yield is about 105 grams (equal to 95 grams of pure material), or about 80 per cent of the theoretical amount.

y acid is analyzed by titration of its strongly alkaline solution with 1 N diazobenzene solution and by titration with nitrite (diazotization) in very dilute mineral acid solution. (General procedures for such analyses are given in the Analytical Section.), The values obtained in

the two titrations should agree within	1 per cent (also in the case of
H acid). If the fusion was conducted	at too low a temperature, the

nitrite titration gives a higher value than the titration with diazotized aniline. The y acid should be at least 91 per cent pure.

(c) 2-Atnino-5-naphthol~7>8ul/onic Acid (J Acid, iso-Gamma Acid)

H03S NH«

5

OH

The procedure is quite similar to that given above for y acid except that it is desirable to use somewhat more water in the fusion mixture — 160 grams instead of 120 grams. The fusion is carried out at a slightly lower temperature, 200-205°C., for 7 hours. The yield of J acid is about equal to that of y acid, i.e.,the equivalent of 95 grams of pure material

208 INTERMEDIATES

(about 105 grams of 92 per cent acid, or 82 per cent of the theoretical amount). Thus, the yield is slightly better for the 2,5,7 acid than for the y acid. Side reactions are not prominent in either fusion since the a sulfo group is much more reactive than the one in the ft position.

The J acid prepared by the above procedure is, in general, sufficiently pure for technical purposes. The product can be purified further by salting out its sodium salt, or by converting it into its difficultly soluble, nicely crystalline zinc salt.

Technical Observations. The small amount of soda admixed with the substance to be sulfonated generates carbon dioxide and thus prevents the formation of hard lumps when the substance is added to the sulfuric acid, The procedure followed in industrial preparations is frequently somewhat different from that given here. Generally, the sulfate or the free 0-naphthylamine is introduced directly into

the 40 per cent oleum. The isolation of the different	sulfonic acids is relatively easy
in the plant, since the separation is more easily	accomplished with the larger

amounts of material involved. Filtration is usually done in wooden filter presses equipped with nitro filters, and the purified acids or their acid salts can be centrifuged. The various mother liquors, which are too complicated in composition to be resolved in laboratory preparations, are worked up either separately or combined, according to their degree of purity. In this recovery, the liquors are completely neutralized with soda and evaporated in multiple evaporators in vacuum until sodium chloride separates. This is removed by centrifuging, and the mother liquor is then returned to the process. In disulfonations of this sort, part of the substance is always lost. Some of this loss is due to decomposition, and some is caused by the formation of easily soluble sulfones or sulfamides which are recognizable by their yellow color.

7 acid and J acid, along with H acid, are the most important aminonaphthol derivatives in the azo dye industry. They are all used in huge quantities in the preparation of wool and cotton dyes. 7 acid is used today in such large amounts that it is manufactured not only from /3-naphthylamine, but also from G acid. J acid has become more important recently and may soon reach the importance of 7 acid, since the azo dyes prepared from it and its numerous derivatives (see Tables XIII and XJV) have very good properties. They are characterized by especially good affinity for cotton, give pure tints, and have, in part, excellent light fastness (see benzo light blue 2GL, page 279 ff.).

The two other methods for the preparation of y acid are Described briefly and schematically in the following paragraphs.

1. From 2-!\aphthol-6,8-di8ulfonic Acid

G salt is converted to amino-G acid by heating with 25 per cent ammonia and ammonium sulfite at 140°C. (pressure, 20 atmospheres). The procedure involves nothing of special interest. A yield of 92 per cent of the theoretical amount is easily attained, and after blowing off the ammonia, the whole reaction mass is fused with caustic.

2.	From G Acid and 2,8>Dihydroxynaphthalene-6-sulfonic Acid
G acid is subjected to fusion with NaOH and a small amount of water at 200°
in an open	vessel to yield 2,8-dihydroxynaphthaIene-6-sulfonie acid. This product

l-NAPHTHYLAMINE-3,6-DISULFONIC ACID ( FREUND ACID)

209

is not isolated, but is treated directly with ammonium sulfate, neutralizing part of the free alkali with sulfuric acid.

SO3H	OH		OH
			NH2
			y- acid
This process gives a very satisfactory product		and has the advantage that
pressures higher than	15 atmospheres are not required. It		is noteworthy that	the
^-hydroxyl group, and not the a-group, is aminated		(cf. page 182).
This process involves the same reactions used		in the	process above, but	in

reverse sequence; alkali fusion first, followed by the Bucherer reaction. In this case, aqueous ammonia is not used, but instead the inexpensive ammonium sulfate which is decomposed by the alkali in the fusion mixture to generate ammonia. Furthermore, the sulfite formed in the fusion reaction is enough to effect the Bucherer reaction.

Phenyl-gamma Acid

OH

i—NH-

A mixture of 239 grams of 100 per cent y acid, 750 cc. water, 750 grams of sodium bisulfite solution containing 25 per cent SO2, and 200 grams of aniline is boiled under reflux for 24 hours. Concentrated soda solution is then added to make the solution distinctly alkaline, and the aniline is removed by steam distillation. On acidification with concentrated hydrochloricacid, the phenyl-y acid is precipitated. The yield is about 270 grams of 90 per cent product, or 75 to 80 per cent of the theoretical amount.

16, l-Naphthylamine-396-disulfonicAcid (Freund Acid)

H03S	SO,H	HO3S	SO3H	HO3S
3,6-Disulfonic		1-Nitronaphthalene-		Freund acid
acid (3,6 =	2,7)	3,6-disulfonic	acid	(1,3,6 = 5,2,7)

In the apparatus described on page 101, 128 grams of pure naphthalene is heated to 165°C. with stirring. During the course of 15min-

210 INTERMEDIATES

utes, 400 grams of 100 per cent sulfuric acid is added, and the mixture is heated for 1 hour at 165°C. to complete the disulfonation. It is not advisable to heat above 165°, or as much as 30 per cent of the 2,6-di- sulfonic acid is formed and this gives a product quite similar to that from the 2,7 (3,6) isomer.

It is relatively simple to determine the extent to which a sulfonation reaction has proceeded. The reaction mixture is neutralized with barium carbonate and treated with enough soda to form the sodium s,alt. The number of sulfo groups present can be calculated from the amount of soda used up. One sulfo group requires 53 grams of Na2COs. Calcium carbonate cannot be used for this purpose since calcium sulfate is soluble in solutions of naphthalenesulfonic acids.

The reaction mixture is now cooled to 15°C., and 103 grams of 62 per cent nitric acid (40° Be) is added over a period of 1 hour, keeping the temperature below 30° since dinitro compounds are easily formed. The solution is held at room temperature for at least 10 hours and is then poured into 1 liter cold water with stirring. The resulting mixture is heated to 70°, and a stream of air is bubbled through it until no more nitric acid oxides are given off. The excess nitric acid is then destroyed by the addition of ferrous sulfate, exactly as was done in the preparation of Cleve acids (page 185), and 150 grams of anhydrous sodium sulfate (or the corresponding amount of the hydrated salt) is added to form the sulfonate. The succeeding steps of removing excess sulfuric acid by means of chalk (about 400 grams), filtering off the calcium sulfate, and reduction, are carried out in the same manner as in the Cleve acids preparation (page 185), with the single exception that here the iron is etched with hydrochloric acid (20 cc. concentrated acid) instead of with acetic acid.

The solution from the reduction is neutralized with soda (testing with ammonium sulfide to be sure that all of the iron has been precipitated) and filtered, then evaporated to 800 cc. and treated hot with 100 grams of potassium chloride. When the latter has dissolved, the solution is acidified at 100° with 100 cc. 30 per cent hydrochloric acid and allowed to cool. The acid potassium salt separates over a period of 12 hours, the whole mass setting to a solid. The pasty mixture is filtered on a large suction funnel with a double filter paper, rinsing the residue into the funnel with the mother liquor and washing the precipitate with 200 cc. saturated salt solution. The product is pressed out as completely as possible in a screw press. The moist press cake weighs 300 to 320 grams, about 270 grams when dry, and corresponds to about 35 grams of sodium nitrite (see page 206).

The mother liquor is dark colored and uses about 22 grams of sodium nitrite, especially if the reduction has not been carried out properly. It contains hydroxyl-

l-AMINO-8-NAPHTHOL-3,6-DISULFONIC ACID (H ACID)

211

amine compounds which give a violet solution on diazotization. The sodium salt can be prepared instead of the potassium salt, by using 100 grams of sodium chloride in the precipitation. The precipitate is much more difficult to filter, however, and the final product is therefore not quite so pure as when the potassium salt is used. By dissolving the press cake in four parts of boiling water and recrystallizing, Freund acid is obtained in a very pure state, which is especially suitable for use in the preparation of complicated polyazo dyes.

17. l-Amino-8-naphthol-3,6-disulfonic Acid (H Acid)

					HO,S NO,
	S03H		H03S
						SO,H
	H03S		NH2	'svr
	H03Si« X $ S03H ~^ HO3S1^			^ ^S03H
(a)	l-l\aphthylamine-396,8-tri8iilfonic			Acid	(Koch	Acid)
In an iron vessel		(or a glass flask or porcelain			beaker with a tight
lead cover)	of about	1-liter capacity, 128 grams of pure				naphthalene

is heated to 150°C., and 140 grams of 100per cent sulfuric acid is added with good stirring, allowing the temperature to rise to 160-165°. The mixing is completed in 10 to 15 minutes and stirring is continued at 160-165° for 30 minutes more. The reaction mixture is then cooled to 100°, an additional 260 grams of 100 per cent sulfuric acid is added in one portion, and the whole is cooled further to 30°. While the temperature is held at this point, 400 grams of 60 per cent oleum is added in the course of 30 minutes, and the mixture is stirred at 25° for 3 hours, then for 7 hours at 165° to complete the trisulfonation (plus some unavoidable tetrasulfonation). The reaction vessel is now placed in ice water and the mixtureis cooled to 10°, 15 grams of ice is added, and 103grams of 62 per cent nitric acid (40° Be) is added dropwise. The temperature must not exceed 10° or extensive oxidation will occur. The nitric acid can be added in about 1 hour. The reaction mixture is allowed to stand overnight, and is then poured into 2 liters cold water. Nitric oxide is given off, and the temperature may rise to 70° without damage. Air is blown through the solution for about 1 hour at 70° to remove most of the nitric oxide, the last parts of which are removed by the addition of a 20 per cent solution of ferrous sulfate (test a diluted portion of the mixture with starch-iodide paper or sulfone reagent; see page 185).